In the new and emerging area of microfluidics, microfluidic carrying devices and their components are small, typically in the range of 500 microns down to as small as 1 micron and possibly even smaller. Such microfluidic devices pose difficulties with regards to preventing fluid path blockage within the microscopic componentry, and especially when the particular microscopic componentry is connected to macroscopic sources of fluid. Yet such microfluidic devices are important in a wide range of applications that include drug delivery, analytical chemistry, microchemical reactors and synthesis, genetic engineering, and marking technologies including a range of ink jet technologies, such as thermal ink jet.
The present invention relates to microfluidic devices in general and in particular to an efficient fluid filtering device for ink jet printers and, more particularly, to a thermal ink jet printhead including such an efficient fluid filtering device.
A typical thermally actuated drop-on-demand ink jet printing system uses thermal energy pulses to produce vapor bubbles in an ink-filled channel that expels droplets from the channel orifices of the printing system's printhead. Such printheads have one or more ink-filled channels communicating at one end with a relatively small ink supply chamber (or reservoir) and having an orifice at the opposite end, also referred to as the nozzle. A thermal energy generator, usually a resistor, is located within the channels near the nozzle at a predetermined distance upstream therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. A meniscus is formed at each nozzle under a slight negative pressure to prevent ink from weeping therefrom.
Some of these thermal ink jet printheads are formed by mating two silicon substrates. One substrate contains an array of heater elements and associated electronics (and is thus referred to as a heater plate), while the second substrate is a fluid directing portion containing a plurality of nozzle-defining channels and an ink inlet for providing ink from a source to the channels (thus, this substrate is referred to as a channel plate). The channel plate is typically fabricated by orientation dependent etching methods.
The dimensions of ink inlets to the die modules, or substrates, are much larger than the ink channels; hence, it is desirable to provide a filtering mechanism for filtering the ink at some point along the ink flow path from the ink manifold or manifold source to the ink channel to prevent blockage of the channels by particles carried in the ink. Even though some particles of a certain size do not completely block the channels, they can adversely affect directionality of a droplet expelled from these printheads. Any filtering technique should also minimize air entrapment in the ink flow path.
Various techniques are disclosed for example, in U.S. Pat. Nos. 5,154,815, and 5,204,690 for forming filters that are integral to the printhead using patterned etch resistant masks. This technique has the disadvantage of flow restriction due to the proximity to single channels and poor yields due to defects near single channels. Further, U.S. Pat. No. 4,864,329 to Kneezel et al. for example, discloses a thermal ink jet printhead having a flat filter placed over the inlet thereof by a fabrication process which laminates a wafer size filter to the aligned and bonded wafers containing a plurality of printheads.
The individual printheads are obtained by a sectioning operation, which cuts through the two or more bonded wafers and the filter. The filter may be a woven mesh screen or preferably a nickel electroformed screen with predetermined pore size. Since the filter covers one entire side of the printhead, a relatively large contact area prevents delamination and enables convenient leak-free sealing. In general, electroformed screen filters which have pore sizes small enough to filter out particles of interest, result in filters which are very thin and subject to breakage during handling or wash steps. Also, the preferred nickel embodiment is not compatible with certain inks resulting in filter corrosion. Finally, the choice of materials is limited when using this technique. Woven mesh screens are difficult to seal reliably against both the silicon ink inlet and the corresponding opening in the ink manifold. Plating with metals such as gold to protect against corrosion is costly, and in all cases, conventional filters ordinarily suffer from blockage by particles larger than the pore size, and by air bubbles.
Conventional filters used for thermal ink jet printheads help keep the jetting nozzles and channels free of clogs caused by dirt and air bubbles carried into the printhead from upstream sources such as from the ink supply cartridge. One common failing of all filters is that dirt can accumulate on the filter surface causing restricted fluid flow. Another kind of blockage is when an air bubble rests on the filter surface thereby covering a large group of fluid flow holes preventing any fluid from passing through that region of the filter.